1. Field of the Invention
This invention relates to a near-field optical probe and a method of preparing the same as well as a microscope, a recording/regeneration apparatus and a micro-fabrication apparatus using the same. More particularly, the present invention relates to a near-field optical probe to be suitably used for a microscope capable of observing micro-structures below the diffraction limit of light, a storage device capable of forming and detecting micro recording bits or a micro-fabrication apparatus capable of producing micro-structures.
2. Related Background Art
In recent years, the resolution of microscopes has been dramatically improved to make it possible to observe specimens as small as a molecule or even an atom by bringing the sharp tip of a probe to the specimen as close as 100 nm or less as a result of the technological development in the field of SPM (scanning probe microscope) typically including STM (scanning tunnelling microscope) and AFM (atomic force microscope).
From the viewpoint of the types of light used for SPM, there have been developed near-field optical microscope (to be referred to as SNOM hereinafter) adapted to observe the surface of a specimen by utilizing the evanescent light oozing out from a micro-aperture arranged at the tip of a sharp optical probe [EPO112401, Durig et al., J. Appl. Phys. Vol. 59, p. 3318 (1986)] and photon STM (to be referred to as PSTM hereinafter) adapted to observe the surface of a specimen by causing light to enter the surface from the rear surface thereof so as to be,totally reflected in the inside and detecting the evanescent light oozing out to the surface of the specimen by means of an optical probe [Reddick et al., Phys. Rev. B, Vol. 39, p. 767 (1989)].
With a SNOM, it is now possible to access a micro-area of 100 nm or less and detect optical information therefrom.
Efforts have been made to develop high density storage devices that surpass the limit of diffraction of light for conventional ones and ultra-micro-fabrication apparatus by irradiating a recording medium or resist with evanescent light emitted from a micro-aperture, utilizing the theory of the SNOM.
Meanwhile, the technique of preparing the optical probe is vitally important for the SNOM. A typical known method of preparing an optical probe for the SNOM comprises melting an optical fiber and extending it along the central axix either by irradiating it with a CO2 laser beam or arranging it between a pair of discharge electrodes, and after sharpening the tip, it is coated with metal from a lateral side while rotating it around the central axis to form a portion having a thin metal film layer at the front end thereof, which is then processed to produce a micro-aperture.
With another technique of sharpening the tip at the front end of an optical fiber, the core is selectively etched to be sharpened by utilizing the difference in the etching rate between the core and the clad relative to the selected etching solution due to the difference in chemical composition.
Evanescent light penetrating through the micro-aperture of an optical probe that is prepared in any of the above described methods and mounted on a SNOM, a storage device or a micro-fabrication apparatus attenuates exponentially as a function of the distance from the micro-aperture, and therefore the optical probe has to be positionally controlled so that the distance between its front end and the specimen, the recording medium or the resist is practically held less than 100 nm.
A shear force control technique or a so-called AFM control technique is popularly used for controlling the above distance. With the shear force control technique, the optical probe is made to oscillate transversally so that the rate of the decrease in the amplitude of oscillation of the optical probe that is caused by the van der Waals force when the front end of the optical probe approaches the specimen is held to be constant. With the AFM control technique, on the other hand, the optical probe is bent to operate as a cantilever so that the amount of flexure of the cantilever caused by the van der Waals force when the front end of the optical probe approaches the specimen is held to be constant.
With the shear force control technique or the AFM control technique, the surface profile of the specimen can be observed, so that highly sophisticated action of an apparatus can be realized when using SNOM signals therewith. For example, from optical and profile information, information on the material of the specimen can be obtained by means of a SNOM apparatus. A storage device utilizing the theory of the SNOM can perform tracking operations using AFM signals. Moreover, a micro-fabrication apparatus utilizing the theory of the SNOM can be used for aligning operations using AFM signals.
The AFM control technique includes a so-called open-loop control technique in which the front end of the probe is held in direct contact with the specimen to make it follow the profile of the surface of the specimen by utilizing the flexure of the cantilever, which is also referred to as contact mode or repulsive force mode. Not only can this technique further simplify the configuration of the apparatus, but it can also make the probe to scan the surface of the specimen at an enhanced rate, because it does not require the use of a feedback loop for controlling the distance between the front end of the optical probe and the surface of the specimen. Then, as a result, it is possible to realize SNOMs that can quickly obtain an observed image of the specimen, high speed recording/reproduction apparatus and high speed micro-fabrication apparatus.
With a known technique that can be used to bend an optical probe for the purpose of AFM control, the optical probe is molten by irradiating a CO2 laser beam to the front end of the optical probe or arranging the optical probe between a pair of discharge electrodes (U.S. Pat. No. 5,677,978; Lieberman et al., Appl. Phys. Lett. 65, p. 648 (1994); Near-Field Nano-Photonics Handbook, ed. by M. Ohtsu, p. 42 (1997)).
In the case of a contact mode AFM, if the specimen, the recording medium or the resist consists of such a relatively flexible material as an organic molecule or a bio-polymer, a large force acts between the probe front end and the specimen, the recording medium or the resist during the scanning operation of the probe to destroy the specimen, the recording medium or the resist surface as well as the probe front end, if the cantilever is rigid. Therefore, the modulus of elasticity of the cantilever in terms of flexure has to be less than 0.1N/m, preferably less than 0.03N/m in order to avoid this problem of destruction.
Optical fibers guide waves of light by utilizing the total reflection at the interface of the core portion having a high refractive index and the cladding portion having a low refractive index. FIG. 16 of the accompanying drawings schematically illustrates the structure of a known optical probe where the front end is made sharp and bent to operate as a cantilever and the core 1401 and the cladding 1402 are coaxially arranged.
Since optical fibers have been developed mostly for the purpose of optical telecommunications using a near infrared laser beam, the core diameter of single mode fibers having a relatively small core is generally between 2 and 10 xcexcm. As a result, when an optical probe is prepared by using such an optical fiber, the outer diameter d of the optical probe will be at least about 10 xcexcm (see FIG. 16).
Optical fibers are mostly made of quartz (SiO2) and the cantilever formed from an optical probe with an outer diameter greater than 10 xcexcm has to have a length L not less than 1 mm as measured from the support thereof in order to reduce the modulus of elasticity in terms of flexure to less than 0.1N/m, when calculated on the basis of the related physical constants of quartz. Then, the resonance frequency of the cantilever will be as low as less than 8 kHz if observed in the direction of flexure. When the resonance frequency of the cantilever is low, the characteristics of the follow-up frequency of the probe relative to the undulations of the surface of the specimen, the recording medium or the resist are lowered to reduce the resolution of observation, to give rise to recording/reproduction errors or to lower the micro-fabrication accuracy.
As a result, it is impossible to realize a high resolution SNOM adapted to high speed observations, a storage device having a high data-transferring rate and a low error rate or a high precision micro-fabrication apparatus with a large throughput.
While a probe with a low modulus of elasticity and a high resonance frequency can be obtained by preparing an optical fiber having a core diameter smaller than 1 xcexcm to form a cantilever optical probe therewith, an optical fiber having a core diameter less than 1 xcexcm cannot practically transmit any light and hence cannot be used for an optical probe because the size of the core is not greater than the wavelength of light.
In view of the above problems, it is therefore the object of the present invention to provide a near-field optical probe having a low modulus of elasticity and a high resonance frequency and a method of preparing the same as well as a microscope, a recording/reproduction apparatus and a micro-fabrication apparatus realized by using such a near-field optical probe and the method of preparing the same.
According to the invention, the above object is achieved by providing a near-field optical probe comprising:
a micro-aperture for irradiating and/or detecting evanescent light through the front end of the probe;
an elastically deformable cantilever supporting the micro-aperture at the free end thereof; and
a surface plasmon polariton waveguide arranged on the cantilever to guide light from a light source to the micro-aperture and/or to guide light from a light source introduced through the micro-aperture.
In another aspect of the invention, there is also provided a method of preparing a near-field optical probe comprising the steps of:
(a) preparing a first probe substrate having a surface plasmon polariton waveguide;
(b) preparing a laser substrate;
(c) preparing a second probe substrate by forming a groove on a substrate followed by forming a probe layer on the groove except the bottom thereof;
(d) bonding said first probe substrate with said laser substrate to form a laser profile pattern thereon followed by forming an insulating layer and an upper electrode;
(e) bonding said first probe substrate subjected to said step (d) and said second probe substrate to transfer said probe layer on said second probe substrate to a bonding layer formed on said first probe substrate; and
(f) forming a cantilever probe having a probe proper projecting from the free end thereof by partly removing said first probe substrate subjected to said step (e).
In still another aspect of the invention, there are provided a near-field optical microscope, a storage device and a micro-fabrication apparatus comprising a near-field optical probe according to the invention.
Now, the present invention will be described by referring to the accompanying drawings that illustrate preferred embodiments of the invention.